feat: add optional Laya engine mode for grouping and language detection (#12)

Add a languageDetectionMode setting (algo | laya | laya+algo) exposed
in Settings with three engine choices. When Laya is enabled, a laya
service (Dockerfile.laya, optional compose profile, models stored
outside the repo under ./data/models/laya) can be started, with model
download and deletion from the UI via /laya routes to manage disk
space. Add a channelLanguageFusion service combining Laya results
with the existing algorithm, and wire detection mode through
grouping, channel list and player. Cover engine selection, fusion
and laya service behavior with tests.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

test/layaService.test.js

@@ -0,0 +1,44 @@
+const test = require('node:test');
+const assert = require('node:assert/strict');
+const fs = require('fs/promises');
+const os = require('os');
+const path = require('path');
+const {
+    LayaRequestError,
+    LayaService,
+    clearDirectoryContents,
+    validateBatchPayload
+} = require('../server/services/layaService');
+
+test('status reports an unavailable runtime without throwing', async () => {
+    const modelDir = await fs.mkdtemp(path.join(os.tmpdir(), 'nodecast-laya-status-'));
+    const service = new LayaService({
+        modelDir,
+        cache: null,
+        fetch: async () => { throw new Error('connection refused'); }
+    });
+    const status = await service.status();
+    assert.equal(status.runtimeAvailable, false);
+    assert.equal(status.modelPresent, false);
+});
+
+test('model deletion remains limited to the configured model directory', async () => {
+    const parent = await fs.mkdtemp(path.join(os.tmpdir(), 'nodecast-laya-delete-'));
+    const modelDir = path.join(parent, 'models');
+    const sentinel = path.join(parent, 'keep.txt');
+    await fs.mkdir(path.join(modelDir, 'nested'), { recursive: true });
+    await fs.writeFile(path.join(modelDir, 'nested', 'model.bin'), 'model');
+    await fs.writeFile(sentinel, 'keep');
+
+    await clearDirectoryContents(modelDir);
+
+    assert.deepEqual(await fs.readdir(modelDir), []);
+    assert.equal(await fs.readFile(sentinel, 'utf8'), 'keep');
+});
+
+test('batch validation rejects oversized item counts', () => {
+    assert.throws(
+        () => validateBatchPayload({ items: Array.from({ length: 65 }, (_, id) => ({ id, text: 'channel' })) }),
+        error => error instanceof LayaRequestError && error.statusCode === 413
+    );
+});